Thursday, November 01, 2007

[FOTHP] Want to See an Exploding Comet?

Want to See An Exploding Comet?

Comet Holmes is a periodic comet that was discovered way back in 1892 by astronomer Edwin Holmes. Since its discovery, the comet has been making regular trips around the Sun, taking a bit under 7 years to make one complete orbit. As far as comets go, Holmes is nothing special. It has been around for a while, has been well studied, and has never been considered a bright or interesting celestial object.

But something spectacular happened just a few days ago. Comet Holmes exploded! Well, maybe not a literal explosion, but something happened to cause Holmes to brighten dramatically over a short period of time. Within a few days, Comet Holmes brightened from magnitude 17 to about magnitude 2. The magnitude scale is set up so brighter objects have smaller magnitudes. Brightening by 15 magnitudes means the comet's light intensity increased by a factor of over one million. That's the equivalent of the moon suddenly becoming much brighter than the Sun. Sounds almost like an explosion, doesn't it? However, unlike stars, comets do not produce their own light; instead, comets reflect light. The brightening of Comet Holmes must be caused by the comet physically expanding and reflecting more light from the Sun. Before the brightening you would have needed a good telescope to spot the comet, but now Comet Holmes is visible to the unaided eye.

Observations have confirmed that the brightening of Comet Holmes has been accompanied by an expansion of gasses surrounding the comet's nucleus, and we are not talking about a small bit of expansion here. According to the folks at, the comet's physical diameter is seven times wider than the planet Jupiter--and it is still expanding. Explosive releases of gasses, known as outgassing, have been suspected on comets for a long time, but no one has actually seen one occur before. It is very likely that explosive outgassing caused the expansion of the comet and the subsequent brightening, but so far no astronomers are willing to commit to that as the best possible explanation. Whatever caused the expansion is still a mystery at this point.

If you would like to catch a glimpse of Comet Holmes then act quickly. No telling how much longer the brightening will last. It is currently in the constellation Perseus and will remain there until the end of the year. If the weather permits, go outside after sunset and look to the northeast. To the unaided eye the comet will appear as a star. With a pair of binoculars or a small telescope you might be able to make out the expanding gas around the nucleus. A finder chart and more recent updates about Comet Holmes can be found all over the web, including at the Hudnall Planetarium website ( One of the best sites for recent updates is the Sky & Telescope website at

This is an exciting time for a not so little comet.

Nov 10 & 11 at 2:00 pm, "More Than Meets the Eye"
Nov 17 at 7:00pm, Public Astronomy Lecture Series
Nov 30 & Dec 15 at 7:00pm, "Star of the Magi"
Dec 1, 2, 8, 9, 15, 16 at 2:00pm, "Star of the Magi"

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